1. Field of the Invention
This invention relates to a method for improving the problem of color-quality loss of a scan apparatus, particularly, to a method for improving the loss of color-quality by adjusting the vertical resolution under low operating resolution.
2. Description of the Related Art
In its early days, the computer was limited to processing objects that did not require high speed, for instance, document processing. As the computer technology advances, the personal computer has been used to calculate data of larger quantity, for example, audio and image data. Because a scanner has the capability of transferring the information on a document to a computer-readable digital data, it has become an important tool in the multi-media era. Generally, the user wants a scanner capable of high-speed scanning and high-quality imaging. Many scanners do offer high-speed scanning process, however, at the lost of image quality.
Please refer to FIG. 1A for the following description of a scanner according to a conventional method. FIG. 1A shows the top view of a scan apparatus 100 and a photo assembly 101 installed in the scan apparatus 100. A document 102 is placed on the platform of the scan apparatus 100. During the scanning process, the photo assembly 101 will be driven by a motor (not shown) and move vertically in the direction V. Next, FIG. 1B shows the cross-sectional view of the scan apparatus 100 of FIG. 1A. The photo assembly 101 comprises a sensing photo module 110, a lens module 104, and a light source 103. When the document 102 is radiated by light emitted from the light source 103, the light is reflected. The reflected light passes through the lens module 104 and focuses at the sensing photo module 110. Then, the photo assembly 101 gradually moves from one end to the other end of the document 102 and the image data of the document 102 is obtained by extracting the photo signal of the sensing photo module 110.
Referring to FIG. 2A, it shows the structure of the sensing photo module 110. The sensing photo module 110 comprises several photo sensors parallel to each other, for instance, a red photo sensor 210, a green photo sensor 220, and a blue photo sensor 230. The optical sensing width for each photo sensor is W and the line distance between the adjacent photo sensors is D. The photo sensors 210, 220, 230 are composed of CCDs (charge-coupled devices) or CMOS sensors for receiving the light signal reflected from the document 102. The optical resolution of the scan apparatus 100, which is also the resolution of the scan apparatus 100 by the horizontal direction, is given by the density of the CCD, whereas the resolution of the scan apparatus 100 by the vertical direction is determined by the velocity of the moving photo assembly 101 and the frequency at which the sensing photo module 110 collects data. Higher velocity of the photo assembly 101 and lower frequency of data collecting for the sensing photo module 110 result in lower resolution along the vertical direction.
The unit of resolution is dpi (dots per inch), which means the number of dots in one inch; more dots in one inch represents higher resolution. Next, FIG. 2B, it shows the positions of the sensing photo assembly 110 for two adjacent data-collecting operations. The data-collecting position of sensing photo assembly 110, represented by the solid line, is processed before the collecting data position of the sensing photo assembly 110, represented by the dotted line. During these two data-collecting operations, the collection distance that the sensing photo module 110 shifts is L. The shorter the collection distance L, the higher is the resolution along the vertical direction. The line ratio is defined as the ratio of line distance D to collection distance L. Therefore, if the line distance D remains constant, higher resolution along the vertical direction can be achieved by decreasing the collection distance L, thus increasing the line ratio.
Please refer FIGS. 3A, 3B, and 3C. They show the moving condition of the sensing photo module 110 relative to the document 102. A red photo sensor 210, a green photo sensor 220, and a blue photo sensor 230, parallel to each other, are equipped in the sensing photo module 110. If the resolution of the sensing photo module 110 is 100 dpi, which is lower than the optical resolution of 300 dpi, for example, then the collection distance L can be selected as 24 μm, and the line distance D can be selected as 48 μm.
The position where the sensing photo module 110 first receives the photo signal is shown in FIG. 3A. As it shifts downward with a collection distance L (24 μm) in the vertical direction, the sensing photo module 110 detects a second photo signal, for which the position is shown in FIG. 3B. FIG. 3C shows the position at which the sensing photo module 110 moves downward for another collection distance L and receives the photo signal for the third time. The sensing photo module 110 will not stop until the entire document 102 is scanned. The line ratio of this example is 2, and the position of the document 102 detected by the blue photo sensor 230 in FIG. 3A is the same as that detected by the green photo sensor 220 in FIG. 3C. For a period of time, the red photo sensor 210 will detect the photo signal at the same position of the document 102. Therefore, after the scanning is completed, one position of the document 102 will have been detected by the red photo sensor 210, green photo sensor 220, and blue photo sensor 230, respectively. The digital data of the image is recovered by processing the red, green, and blue photo signals.
However, in the case of low resolution due to long collection distance L, a 110 certain position of the document 102 will not be detected by all of the red, green, and blue photo sensors because the line ratio number is not an integer. Accordingly, the red, green, and blue images do not overlap and the image loses its quality.